Clarification in DLQ documentation (apache#5029)

Clarification in DLQ documentation.

site2/docs/concepts-messaging.md

@@ -100,7 +100,7 @@ When a message is not consumed successfully, and you want to trigger the broker
 
 Dead letter topic enables you to consume new messages when some messages cannot be consumed successfully by a consumer. In this mechanism, messages that are failed to be consumed are stored in a separate topic, which is called dead letter topic. You can decide how to handle messages in the dead letter topic.
 
-The following example shows how to enable dead letter topic in Java client.
+The following example shows how to enable dead letter topic in a Java client using the default dead letter topic:
 
 ```java
 Consumer<byte[]> consumer = pulsarClient.newConsumer(Schema.BYTES)
@@ -113,7 +113,28 @@ Consumer<byte[]> consumer = pulsarClient.newConsumer(Schema.BYTES)
               .subscribe();
 
 ```
-Dead letter topic depends on message re-delivery. You need to confirm message re-delivery method: negative acknowledgement or acknowledgement timeout. Use negative acknowledgement prior to acknowledgement timeout. 
+The default dead letter topic uses this format: 
+```
+<topicname>-<subscriptionname>-DLQ
+```
+
+If you want to specify the name of the dead letter topic, use this Java client example:
+
+```java
+Consumer<byte[]> consumer = pulsarClient.newConsumer(Schema.BYTES)
+              .topic(topic)
+              .subscriptionName("my-subscription")
+              .subscriptionType(SubscriptionType.Shared)
+              .deadLetterPolicy(DeadLetterPolicy.builder()
+                    .maxRedeliverCount(maxRedeliveryCount)
+                    .deadLetterTopic("your-topic-name")
+                    .build())
+              .subscribe();
+
+```
+
+
+Dead letter topic depends on message re-delivery. Messages are redelivered either due to [acknowledgement timeout](#acknowledgement-timeout) or [negative acknowledgement](#negative-acknowledgement). If you are going to use negative acknowledgement on a message, make sure it is negatively acknowledged before the acknowledgement timeout. 
 
 > Note    
 > Currently, dead letter topic is enabled only in the shared subscription mode.